1. Technical Field
The present invention relates to a key phone system accommodating both an integrated service digital network (ISDN) and a public service telephone network (PSTN), and relates more particularly to a multiple subscriber service implementing method for a call reception service and a call transmission service in a key phone system accommodating multiple subscribers through either an integrated service digital network (ISDN) line or a public service telephone network (PSTN) line.
2. Background Art
Generally, a key phone system includes a plurality of telephone sets and a central unit adapted to selectively set up a connection between one of the telephone sets and a selected one of a plurality of central office lines. Each of the telephone sets is provided with a hold button and a plurality of sets of central office lamps and buttons, each set corresponding on a one-to-one basis to the central office lines. The central office lamps allow the user to visualize the status of each individual central office line. A central office lamp turns off while the central office line is idle, turns on while the central office line is in use, flashes slowly while the central office line is being held, and flashes rapidly while an incoming call is being received over the central office line. The central office buttons are available for selecting and seizing any of the central office lines. The central unit detects an operation of the central office buttons which may occur at any telephone set so as to connect the telephone set to the selected central office line. Examples of such a key telephone system are disclosed in U. S. Pat. No. 4,682,353 for Key Telephone System issued to Inoue et al., U.S. Pat. No. 5,513,256 for Key Telephone System issued to Komuro, and U.S. Pat. No. 5,544,231 for Conversation Recording/Playback Method In A Key Phone System issued to Cho.
In a traditional public service telephone network (PSTN), central office lines are conventional analog lines. As a result, only a single telephone call can be transmitted or received via a single central office line. In recent years, however, an integrated services digital network (ISDN) allows a single central office line to transmit or receive a plurality of telephone calls such as disclosed in U.S. Pat. No. 5,182,768 for Digital Telephone Set Capable Of Being Simultaneously Used By A Plurality of Persons By Using One Transmission Line issued to Nakano et al. The emergence of integrated service digital network allows the key phone system to accommodate both the digital and analog telephone networks in a manner such as exemplary disclosed in U.S. Pat. No. 4,958,369 for Key Telephone System Accommodating Both Digital And Analog Telephone Networks issued to Tsuchida, U.S. Pat. No. 5,070,524 for Telephone System issued to Mano, and U.S. Pat. No. 5,121,424 for Telephone System And Speech Level Adjusting Method Therefor issued to Mano. For example, Tsuchida '369 discloses a key phone system which allows the user to perform different calling transmission operations in accordance with the type of central office line acquired. A display unit is provided to inform the user whether the central office line is either an integrated service digital network line or a public service telephone network line so that the user can fully utilize various functions available in the integrated service digital network. Mano '524 and '424 seek to automatically regulate the transmission speech loss irrespective of whether the central office line is either an integrated service digital network line or a public service telephone network line. However, it has been my observation that none of the conventional key telephone systems discloses a simple and yet effective way to provide a call reception service and a call transmission service for both the integrated service digital network and public service telephone network.